Ultrasound imaging is a non destructive and noninvasive way of examining structures concealed from ordinary visual inspection; e.g., organs within a living creature, such as the human body. For an appreciation of the overall environment in which the invention is of benefit, refer now to FIGS. 1A-C, wherein an ultrasound imaging unit 1 is shown having an untrasound probe 2 coupled thereto by a cable 3 for forming an image (not shown) upon a CRT 4 of an organ 5 beneath an exterior layer of skin (also not shown). To do this the probe 2 is brought into contact with the layer of skin. Ultrasonic transducer elements (7, 8, . . . 9) within the probe 2 emit "beams" of ultrasound 6. A beam is a short burst of ultrasonic acoustic energy traveling along a direction in which the probe 2 is generally oriented. The ultrasound beams are emitted one at a time. A given beam 6 may be emitted several times in succession prior to the emitting of an adjacent beam, which in turn may be emitted several times, and so forth. The acoustic energy in a beam 6 is partially absorbed by tissue within the body as it propagates inward toward the organ 5. Some is absorbed by the organ, while some portions are reflected. Some of what is reflected travels back toward the probe 2 (return beams 10-15), where signals generated in the transducer elements that can be processed to contribute to the formation of an image corresponding to the shape of the organ 5. Some ultrasound systems are equipped to detect a doppler shift in the frequency of the reflected ultrasound energy. The doppler shift is caused by motion within the substance doing the reflecting, e.g., blood within a vessel or the heart. In these systems the doppler shift corresponds to a velocity of interest, and different velocities are represented by different colors.
The more sophisticated ultrasound imaging systems use a phased array of transducers (7, 8 . . . 9) in the probe 2 to steer the ultrasound beam 6 in a direction of interest. For example, the beam 6 depicted in FIG. 1A is not perpendicular to the row of transducers 7-9 in the probe. This steerability is achieved by exciting the transducers 7-9 in the probe 2 at slightly different times, such that cancellation and reinforcement of the outward propagating ultrasound concentrates the ultrasound energy in the desired direction. The various directions are referred to as radials, and if an odd number of transducers were excited to produce a beam along a particular radial the beam or radial is said to be centered on the middle transducer of that odd number. An even number of transducers would produce a beam or radial centered on the midpoint between the two innermost transducers of the even number thereof. The number of transducers excited to produce the beam along a given radial is called the aperture, and the number of transducers in the aperture is generally less than the total number of transducers in the probe 2. As a consequence, the aperture is frequently (although not always) shifted across the surface of the probe 2 as the imaging operation proceeds. (Those familiar with ultrasound imaging equipment for medical applications will recognize that the field of view 16 depicted in FIG. 1A corresponds to a non-shifting aperture.)
Steerability of the beam may be thought of as focusing, and it works also for ultrasound reflected from things within the field of view 16 and subsequently propagating back toward the probe. To focus in the receive case the signals produced by the various transducers in the aperture are selectively delayed and then summed. This allows the probe 2 to listen with much greater sensitivity to reflections originating along a given radial.
The focusing that steers the transmitted beam or sensitivity to reflections actually focuses to a point along the desired radial, rather than to the entire radial itself. In many instances the depth of field is such that adequate definition, or resolving power, is maintained by setting the transmit focal point at some fixed depth. In other instances it is possible to change the transmit focal depth to be either shallower or deeper. During receive the focal point can be continuously adjusted to lie at different depths along the radial upon which the most recent ultrasound burst has been transmitted. This variation in receive depth focusing is performed in real time as the various echoes are returning from along that radial.
The ability to steer the beam allows the field of view 16 to be a sector-shaped section, rather than a simple parallel projection outward from between the transducers on the ends of the probe 2. Steerability can also be used to significantly increase definition and resolving power of items of interest that are squarely within the field of view.
As will be appreciated as the discussion proceeds, each transducer has associated with it an individual signal path that contains a great many other items, such as delays, amplifiers and perhaps switches. Each such signal path and its associated collection of hardware (including the transducer) is commonly referred to as a "channel".
To continue, the ultrasound unit 1 both controls the excitation of the transducer elements within the probe 2 and does the processing necessary to combine the echoes (reflections) into an image. To this end the various transducers (7, 8, . . . 9) in probe 2 are coupled to respective transmit drivers (20, 21, . . . 22) and to respective receive amplifiers (23, 24, . . . 25). A respective collection of transmit delay elements (26, 27, . . . 28) are programmed via a control bus 29 to delay a FIRE.sub.-- LINE signal 30 by amounts selected to focus the energy of each transducer in the aperture onto a point along the desired radial. The variously delayed FIRE-LINE signals are coupled to their respective transmit drivers (20, 21, . . . 22) whose outputs are high voltage pulses at the desired frequency and which are conveyed by the cable 3 to the transducer elements (7, 8, . . . 9).
When thus excited each transducer produces a short burst of sinusoidal compressional motion that propagates acoustically into the body. Typical frequencies for the acoustic energy are 2.5, 5 and 10 MHz. The focused beam 6 propagates outward from the probe 2 along the desired radial. As different features are encountered various partial reflections are generated. In the figure, for example, dotted lines 10, 11, . . . 12 represent acoustic energy reflected, from the near side of a blood vessel 5, to corresponding channels 1, 2, . . . n. The dot/dashed lines 13, 14, . . . 15 represent acoustic energy reflected a bit later in time from the far side of the blood vessel 5.
Immediately after the transmitted burst reflected acoustic energies reaching the various channels become electrical signals that are amplified by the corresponding receive amplifiers 23, 24, . . . 25. The outputs of the receive amplifiers are coupled to additional corresponding amplifiers 31, 32, . . . 33. To counteract the greater attenuation experienced by echoes originating deeper within the body gain of these additional amplifiers is ramped upwards during receive time, until a limit is reached where the noise floor becomes obtrusive. This ramping is accomplished by gain control line 34. The outputs of these amplifiers are converted to a more convenient intermediate frequency (not shown) and coupled to associated respective programmable delay elements 35, 36, . . . 37. The delayed signal for the channels in the aperture are then summed in a summer 38.
At this point the signal emerging from summer 38 may be likened to an IF signal that possesses a rather complex form of modulation. The center frequency of the IF signal represents (but is not the same as) the frequency of the original sinusoidal burst produced when the transducers were excited at the time the (radial) line was fired. The amplitude represents the degree of reflection from things along the radial, and in the absence of any significant reflection the amplitude may approach zero. The amplitude varies with time as reflections along the radial, occur at different depths. However, since reflections originating deeper within the body are subject to attenuation of the original burst as it propagates outward away from the probe and then again as the reflection propagates back toward the probe, an attempt is made to normalize amplitude by ramping the gain of the receive system as a function of time. This makes amplitude a function of reflection coefficient (of the organ 5) rather than of the depth of where that reflection coefficient occurs. Any deviation in signal frequency from the center frequency indicates motion within the thing doing the reflecting.
Just as in most receivers, it is the modulation that conveys the useful information. What is wanted now is to extract the amplitude variation information and the frequency variation information, and dispense with the IF component. This is accomplished in a known way by mixing (heterodyning) the carrier with local oscillator signals 41 and 42 that are each the same in frequency as the IF but out of phase with each other by ninety degrees. The local oscillator signals and the IF signal are mixed in mixers 39 and 40 to produce two baseband signals I 43 and Q 44 that together represent the information that is of interest. They are still AC signals (even though we have "mixed them right down to DC") representing certain useful information, although they are not signals that represent that information in its most irreducible form. What is more, signals I 43 and Q 44 must be treated as an inseparable signal pair, in that both are now needed to recover that useful information.
Before passing to FIG. 1B we should note that the system architecture has thus far been described in largely functional and conceptual terms rather than as a literal description of a particular way of implementing these functions. For example, an actual system may have coarse and fine delay, and first and second intermediate frequencies, and some rather messy interactions between all these pieces. A detailed description of all that has been omitted here for the sake of brevity in exposing the central concepts that are of interest further on below.
Progressing now to FIG. 1B, the signals I 43 and Q 44 are each converted from timevariant analog signals to sequences of digital values representing corresponding instantaneous values of the analog signals. These conversions are performed by A/D converters 45 and 46, respectively. The resulting digital values (47 & 48, respectively) are stored in a memory 49.
As above, our description and explanation of the memory 49 are functional and conceptual. It will be appreciated that the input switch 50 and the output paths 51 are the result of the addressability of the memory and of the buffering of values read from the memory 49. Likewise, the memory 49 isn't really fundamentally formed of columns (52, 53, . . . 54), where each column is conveniently a stack of partitioned bins for receiving pairs of I and Q values. Instead, it is just a random access read-write memory (RAM) that has some structure imposed onto its address space by a state machine (or perhaps a microprocessor) that controls (among other things) the storage and retrieval of data into the memory 49.
It will be recalled that a "line" along a given radial might be "shot" several times before the steering is changed and a new radial chosen. There are various reasons for this, among them the desire to average out noise and, in a doppler system, to dwell on the current radial for at least two shots to gather velocity data for the different depths along that radial. If a sufficient number of shots can be made along the radial, it is possible to identify turbulent flow by rapid changes in velocity at a given location. With this in mind it will be appreciated that the various columns (52,, 53, . . . 54) are each for different (consecutive) shots along the same radial. The columns store I and Q values as pairs. The tops of the columns are for the I and Q value pairs that are of minimum depth, and the bottoms of the columns are for I and Q value pairs at maximum depth. As a pictorial device, we have drawn the figure as showing the different columns as slightly offset vertically, so that non-overlapping outputs 51 can originate from different columns but still represent the same depth; the memory itself has no such funny business.
Let there be M-many column (52, 53, . . . 54); typical values for M are in the range of, say, four to twelve. Accordingly, there are M-many output IQ value pairs 51 for each depth. These output IQ value pairs for a given depth are referred to as a "packet". The various components of a packet convey useful information. To begin with, each IQ pair is understood as legs of a right triangle enclosing the right angle. The resulting hypotenuse (averaged over the entire packet) is the (normalized for depth) amplitude of the corresponding reflected echo, is called magnitude, and will be denoted as PKT.sub.-- MAG. One of the acute angles is the phase of the baseband signal represented by the analog I and Q signals, 43 and 44. Velocity for the packet is the (averaged) change in phase with respect to time (i.e., from IQ pair to IQ pair within a packet), and will be denoted as PKT.sub.-- VEL. A final parameter of interest can be determined, and it is a variance that is a measure of differences between the various velocity values in the packet. A high variance can be taken as an indication of turbulence. The variance parameter will be indicated as PKT.sub.-- VAR.
Refer now to FIG. 1C. The M-many outputs 51 for the current depth along the current radial are coupled to a clutter filter portion 56 of a packet processor 55. The clutter filter 56 is principally a high-pass filter that can remove certain extreme indications in the data, such as reflections from stationary objects. Following the clutter filter the data is applied to a magnitude detector 57 whose output is PKT.sub.-- MAG 58, to a velocity detector 59 whose output is PKT.sub.-- VEL 60, and to a variance detector 61 whose output is PKT.sub.-- VAR. Regardless of what M is, there is but a single instance of each of these PKT signals for each depth. The order in which these three PKT signals (58, 60 and 62) is produced is that of increasing depth. The detectors 57, 59 and 61 could be software routines executed by a microprocessor, if it were fast enough. The preference at the present time, however, is to use dedicate high speed logic (e.g., state machines formed of PLA's) programmed specifically for these tasks. The three output values PKT.sub.-- MAG 58, PKT.sub.-- VEL 60 and PKT.sub.-- VAR 62 are applied to a detection strength spatial filter 63. The output of the spatial filter 63 is a refined notion of velocity and variance, which when used in place of their unrefined counterparts at the input to the spatial filter 63, produce an enhanced image on the CRT 4.
The bulk of our interest from here on will be what goes on inside the detection strength spatial filter 63. To do that it will be necessary to appreciate basically what a simple spatial filter is and why it is wanted in the first place. Its fundamental component is a subsector memory, of say, four or five banks. Each bank can store an entire radial line's worth of packet magnitudes, packet velocities and packet variances. That is, those three things for all the different depths along that radial, say 512 or 1024 different depths. The different banks store consecutive radials, so that the stored data describe a contiguous sub-portion of the field of view. Hence the term "subsector memory". In brief, the subsector memory allows acquired data that is time variant (think: "is sequentially organized") along individual radials to be filtered in each of two dimensions in which the displayed image will be presented: a radial dimension and a lateral dimension. The lateral dimension is in the direction of to an adjacent radial while remaining at the same depth. Note that the acoustic echo detection mechanism is inherently a radially oriented technique, and does not of its own accord produce any time variant signals where time corresponds to the lateral dimension. Once data is in the subsector memory, however, it can be traversed along the lateral dimension to produce just such a sequentially organized sequence of signal values that can indeed be filtered, just as the time variant radial dimension signal can be filtered. This two dimensional spatial filtering appears as a smoothing of the images and a reduction in noise.
As powerful as the notion of a spatial filter is, merely having one is not the last word on image enhancement. A great deal depends upon the strategies of the filters applied to the data passing through the subsector memory. Magnitude is not generally filtered. Merely filtering velocity as an independent variable can produce a blood flow image that could still benefit from further enhancement. To provide that further enhancement we shall introduce the notion of "detection strength." Detection strength is a mapping of (magnitude, velocity) pairs representing the likelihood that the instant pair describes flow, as opposed to noise or perhaps clutter. Consider, for example, a cardiac image with doppler detection of blood flow. A low velocity and large amplitude are likely to be the wall of the heart, while a low velocity and a small amplitude are likely to be noise. High velocity and large amplitude are very likely to be blood flow, while high velocity and very small amplitude is likely to be noise. In-between are a range of possibilities, which is matched by the range of the assignable values for detection strength. By assigning a detection strength to each velocity and then sending both (along with the associated variance,too) through the spatial filter, velocities that are produced by actual flow are more likely to be correctly identified as such. The detection strength mapping may be adjusted according to the particular imaging application at hand.
To continue our overview of an ultrasound system of the type that is of interest, the filtered outputs PIX.sub.-- VEL 64 and PIX.sub.-- VAR 65 are applied to a scan converter 66, where the coordinate system is changed from being by depth along consecutive radials to a more convenient (X, Y) arrangement that lends itself to a raster scan display. The converted data is then stored in a frame buffer 67, after which a color map 68 allows the assignment of different colors to different values of selected kinds of data. The result is a stream of RGB signal data that is applied to the CRT 4 (along with horizontal and vertical sweeps signals, which for brevity we omit), and the subsequent formation of an image thereon.
The image may appear upon the CRT 4, although other displays and various hardcopy output devices are certainly possible. A collection of controls (not shown) allows the operator to adjust various parameters, such as gain and the filtering parameters, to obtain the most useful image.
We turn now to another topic that will be of interest. There is a mode of operation called "parallel flow" where (referring again to FIG. 1A) a given radial line is shot just as before, but twice as many receive channels are used in forming the image. The capability of doing this can be used to cut the number of transmit lines in half, while keeping the number of existing receive lines. This has the effect of nearly doubling the rate at which data is acquired and displayed. The parallel flow capability can also be used to keep the same number of transmit lines, while doubling the number of receive lines. This results is an image having greater resolution. In either use there sometimes occurs an annoying artifact called "corduroy". There follows now a brief description of how parallel flow is accomplished, and of how the corduroy artifact arises.
The starting point for understanding parallel flow is to note that if there are n-many transmit channels in use, then they are accompanied by 2n (or perhaps 3n, 4n)--many receive channels. Because of the large numbers of channels involved, practical systems currently limit n to 2. By a transmit channel we mean the transmit delay, driver and transducer involved in generating an acoustic impulse of ultrasound. For example, in FIG. 1A transmit ch. 1 would include delay 26 driver 20 and transducer 7. By a receive channel we mean the transducer, amplifiers and receive delay associated with responding to an ultrasound echo. For example, receive ch. 1 would include transducer 7, amplifiers 23 and 31 and receive delay 35. Note that transducer 7 belongs to both transmit ch. 1 and receive ch. 1. Associated with a group of receive channels is a common collection of other hardware, such as a summer, mixers and A/D converters.
To proceed by way of an example, suppose that there were sixty-four transmit channels available, and let them be denoted T1 through T64. We now require two groups of receive channels. Let these be R1 through R64 and R65 through R128. R1 through R64 are just as shown in FIG. 1A, where Ch. N is Ch. 64. Now provide a duplicate set of that same hardware, including their associated summer, mixers and A/D converters. (The extra A/D's can be supplied, or the existing ones can be multiplexed, which is cheaper.) This duplicate set has sixty-four inputs that correspond to the inputs to amplifiers 23, 24 . . . 25, and two outputs that correspond to digital values 47 and 48 (shown on FIG. 1B). (If the extra A/D's are not supplied and the existing ones are multiplexed, then the existing digital value 47 alternates between a oh. 1-64 value and a oh. 65-128 value.)
The input channels are connected in parallel in the following way: ch. 1 and ch. 64, ch. 2 and ch. 65, . . . , ch. 64 and ch. 128. The steering or focusing for the two set of channels is arranged as follows: if the transmit channels shoot a line along radial N, then one group of receive channels receives along "radial N-1/4 while the other group receives along "radial N+1/4. (While correct in spirit, this is a bit of a simplification. The naked truth is rather ugly, and involves complications arising from finite apertures and long wavelengths, and has been happily omitted in favor of this slight simplification.) The result is twice the number of received radials as compared to non parallel flow operation. To use this resource to provide improved resolution, the incremental angle between received radials is cut in half, thus providing twice the number of radials for the same field of view. The data from the two groups of channels goes into the memory in the obvious way; as consecutive radials interleaved to be in their correct order.
The problem arises when the two groups of receive channels do not have exactly the same gain. True, they can be matched to a point, typically to within a db or so, on average. But it must be remembered that the groups would have to track gains not only laterally, but for all depths, too. Peak gain variations might be a few db at certain places in the image. Now, there is of necessity a thresholding process which blanks to black any data that is deemed too weak or too noisy to display. This means that it is possible for one group of channels to produce for consecutive radials data that for a range of consecutive depths is above the threshold while the other group does not, solely because the gain difference produces data that straddles the threshold. When this happens, the display possesses, at that range of depth and for those radials, a radial-by radial picket fence like quality which is what we have termed "corduroy". That term is used because it suggests a striped quality in the display. To complete the picket fence analogy, the pickets would be normally colored radial regions for the detected velocity, while the space between the pickets (intervening radials belonging to the other signal path having the lower gain) would be black. The height of the pickets might be, for example, in the range of from five to twenty-five percent of the length of a radial, and could exist laterally for a large number of consecutive radials; accordingly, the effect can be quite annoying.